Hi guys

Apologies I dont speak German so writing in English

I have built a standalone unit Dumbleator powered by the Vpump. I have added the Mix Pot 500KB variation as per the Dumbleator's schematic but I have a bad ground loop hum/buzz.

I am trying to understand how to ground the project. The PCB has a ground hole next to the Sout, and RGround hole and a Ground hole.I have used the Ground hole to attach to the shield of the output socket. This output socket is the only place which touches the case to ground it. The Sground attaches to one side of the Send pot and the R ground is not connected.

Can somebody tell me where I have gone wrong and why the ground loop is occurring?

Thank you

Can someone tell me If I should do anything with the Ground

Zitat von: OrangeDrop am Jul 15, 2023, 12:34
Hi guys

Apologies I dont speak German so writing in English

I have built a standalone unit Dumbleator powered by the Vpump. I have added the Mix Pot 500KB variation as per the Dumbleator's schematic but I have a bad ground loop hum/buzz.

I am trying to understand how to ground the project. The PCB has a ground hole next to the Sout, and RGround hole and a Ground hole.I have used the Ground hole to attach to the shield of the output socket. This output socket is the only place which touches the case to ground it. The Sground attaches to one side of the Send pot and the R ground is not connected to anything.

Can somebody tell me where I have gone wrong and why the ground loop is occurring?

Thank you

Hi OrangeDrop,

i am assuming you are running this stand alone? Whats the exact configuration here? 

Guitar -> Dumbleator -> Amp ?
Then where is the Vpump getting voltage from?

It would help alot, if you post some images of your build.

Cheers,

Geronimo

Ah thanks Geronimo.

I think I've realised where my problem is - its not the Dumbleator at all but rather my amp and where I've tried to insert the Dumbleator between the preamp and power amp. The buzzing is crazy, yet when I try another amp there is no problem at all! 

I suspect my Fender AB763 circuit is either not grounded properly, or simply cutting between the preamp and power amp of this circuit doesn't allow for an effect loop like the dumbleator.

I'm guessing a discussion about the amp itself and where to insert this dumbleator fx loop is probably beyond the scope of this forum? 

Hi,
btw - what is your real name?

Most propably you create a groundloop by connecting the dumbelator with the amp. Both of them have 3 prong mains cord with PE - right?
Hopefully no ground swithces and no death caps.

Some amps  are more intolerant for ground loops as others...  E.G. my 76 vibrolux is a finnicky best in regards of that.
You might isolate the dumbelator by using two line level isolating transformers in the send and return thus interupting the loop.



Don't cut the PE at any of the units under no cricumstances.


regards
Jochen

Zitat von: OrangeDrop am Jul 17, 2023, 07:43
Ah thanks Geronimo.

I think I've realised where my problem is - its not the Dumbleator at all but rather my amp and where I've tried to insert the Dumbleator between the preamp and power amp. The buzzing is crazy, yet when I try another amp there is no problem at all! 

I suspect my Fender AB763 circuit is either not grounded properly, or simply cutting between the preamp and power amp of this circuit doesn't allow for an effect loop like the dumbleator.

I'm guessing a discussion about the amp itself and where to insert this dumbleator fx loop is probably beyond the scope of this forum?

Grounding is often a very misunderstood topic.

There are some different variants aswell as different solutions to each of them. If any applies on your Fender Amp - only you know.

Usually the ground starts at the lowest impedance - which typically is the inlets earth wire - which should always follow directly to chassis. Then from there your actual circuit GND starts - so we see this point as lowest and therefore "cleanest" ground.

Then the most common variant is a daisychained ground.
Meaning that from that chassis point one wire follows to the actual circuit ground (highest current stages first - final stage the preamp) and goes further from stage to stage.
If you break that ground to daisychain a new stage in there, then you need to make sure that the following rues are met.

- The Ground of your stage only carries same type of signals as the original ground was inteded to
  * Meaning in your case: It only carries an audio ground. If you also add a voltage ground there for lets say a tube filament or a relay or what not, then you sucessfully dirtied that ground and therefore this can cause humming.

- The Ground that you broke must be connected back together via the device you added in
  * if you dont make sure about that then any following stage might be ungrounded which is potentionally harmful

Typical humming cause:
- One or more stages arent grounded at all (open loop)
- One or more stages have two paths to ground (ground loop)
- One or more stages have ground to a dirty potential

The dirty potential is whats actually the most common problem. Even a tiny amout of current of a relay coil can rise a ground to a few uV or even mV, depending on the actual current and the resistance between the cleanground and that point. This is considered a dirty ground then. Once you ground your audio stage there, it injects a hum due to a voltage offset. You will have an immediate hum impact due to the high impedance and gain of your application.
Thats why some designers prefer Stargrounding which is not the ultimate solution for every question in grounding.

In case of the dubmleator (i have never build one myself) all you really need to check is if the "S GND" is connected to the "R GND"
If not you should just simply run two wires from each ground to the Circuit ground near the stage that you loop in to.

Then for a traditional Fender Circuit you wont need a charge pump at all. You just simply need to connect the B+ of the Dumbleator to the volage source that feeds the stage that you hack into  (if i am not mistaken) - The project page might cover where to get sourced from.


Hope this helps.

Cheers!

Geronimo

Thanks Jochen and Geronimo

My name's Grant:)

So to clarify:

1) The dubmleator kit I used is stand alone and housed in its own little metal chassis case. It is powered by a VPump. Accordingly, it is powered by a simple 12vdc plug pack wall wart adapter. As it is DC powered I figured the VPump circuit didn't need to be attached to chassis or anything. I simply connected the high voltage output of the VPump to the Dubleator's B+ input and -input.

2) The dumbleator's PCB Rground, Sground, and Ground all seem to be connected on the PCB itself. I had always been told (and Geronimo your explanation of different grounds was very helpful thanks) - that there should be only one audio star ground point (with the exception that if a circuit is powered by a 240vac transformer the mains power should have its own earth point on the chassis).  So Given this, as Rground, Sground and Ground were all connected on the PCB, I simply connected at one of these points and attached this to the sleeve of the dumbleator's output socket that touches the chassis.

3) The fender circuit I used was a Deluxe Reverb. I am not sure if its possible to attach schematics in this forum and I couldn't find a way to attach one but the fender AB763 circuit has a point where a 0.1uf capacitor comes out of the preamp section and meets a 0.001uf coupling capacitor which is the beggining of the phase inverter. I cut the circuit at the connect of these two caps in the amp and installed two 6.5mm jacks into the back of the amp - ie an output of the 0.1uf preamp capacitor being the "send" and the input of the 0.001uf coupling phase inverter cap being the "return" socket. Ie I installed a passive effects loop at these points. From there I plugged the dubmleator into the amp via tow guitar pedal patch leads.

This setup has created the ground loop. Even as I touch the guitar strings the buzzing sounds changes. I'm just not sure what I'e done wrong in the fender circuit to create this ground loop.

Thanks for all your help. I am very grateful:)

Hey Grant!

Yes of course the S and R GND's are tied together on the pcb itself. I just was not sure because i never built one myself. According to the schematic it says RGND and SGND. Having two different signal names for technically one and the same signal was a bit confusing to me.
I think this was made for simplicity. Because if you tie in a new stage to a daisychained gnd application then you might want to break the ground of that particular stage to feed it through your newly added stage. This of course is not possible if you have the new stage in an external housing. 

Anyway. I think its hard to tell where the humming may come from without seeing the actual build itself. Having the Dumbleator in an external case with its own supply adds the problem of having wires running to your amplifier. Wiring adds huge inductance and parasitic resistance and any of these can ruin the grounding idea of the amplifier. One Amp might be more forgiving than the other.

Maybe you are lucky finding someone who successfully uses the Dumbleator in a AB763 circuit.

Have a great time!

Geronimo

Thanks for all your advice Geronimo. I appreciate it.

I suspect you are right that adding wires to the amp creates further problems in terms of noise picking up etc. I guess I'll just keep experimenting or use my other amp that seems to not have hum.

Cheers
Grant

Hi Grant,
so you don't have a mains ground loop, as the wallwart is isolated from the PE. That's good.

Is the PE of the Fender working properly?

What happens if you connect just one cable (send or return) to the dumbelator? Any changes?


regards
Jochen